Fluid composition sensor using reflected or refracted light monitoring

ABSTRACT

A gas density sensor having a prism in contact with a fluid whose density is determined. A light source shines light into the prism. The light is reflected off prism surfaces in contact with the fluid. As the fluid density changes, the amount of light reflecting off these surfaces changes depending upon fluid density. A detector placed to receive light reflecting off the surfaces determines density from sensed light.

FIELD OF THE INVENTION

The present invention concerns method and apparatus for measuring fluidproperties such as gas density, gas or liquid chemical composition,physical state, temperature, or pressure.

BACKGROUND ART

This invention concerns a method of directly measuring a physicalproperty of a fluid such as gas density and/or using the measuredphysical property to determine another property such as gas pressure.Conventionally, gas density can be determined by weighing a gascontainer, measuring its volume and subtracting the container's emptyweight. The weight difference is then divided by the container's volumeto obtain the density. A major difficulty resides in trying to measurethe filled container's weight, especially when the container is mountedfor use. Gas density can also be determined by applying gas law data toa measured pressure. This requires an accurate pressure and temperaturemeasurement, and an accurate determination of the gas law for themeasured gas. The gas law in turn will vary from gas to gas and atdifferent combinations of pressure, density and temperature.

Gas pressure measurement also presents challenges. At the present time,one of the most common methods of gas pressure measurement is theBourdon Tube. This simply consists of a bent or coiled tube closed atone end with the other end open and mechanically fixed to the systemwhose gas pressure is to be measured. The outside of the tube is kept ata reference pressure (usually atmosphere). When the pressure inside thetube exceeds the pressure outside the tube, the tube will began tostraighten. When the pressure inside falls, the tube will began toreturn to its bent form. Typically, a device such as a hinged meterneedle, is attached to the closed end of the Bourdon tube. As the tubemoves the needle then moves along the meter scale. By appropriate choiceof materials and tube dimensions, it is possible to achieve a meterindication that is approximately linear or at least smoothly varyingwith respect to pressure variation. The disadvantages of the Bourdontube reside in the difficulty of repeatably reproducing the samemechanical behavior from tube to tube. This results in a need to eithermechanically calibrate each tube or accept a large variation in pressureresponse from tube to tube. The motion of the tube is also susceptibleto temperature variation. As the tube heats its thermal expansion willcause extension and its other mechanical properties will also vary.Additionally, a mechanical linkage is required to whatever is used as ameter. Mechanical linkages necessarily exhibit hysteresis, wear and`play`.

Another common measuring technique is to place a strain gauge on adiaphragm. Strain gauges can be composed of materials such resistivefilms or piezoelectric elements, among others. The diaphragm in turn isfixed at the edges and is placed between the pressure to be measured anda reference pressure. As the pressure difference between the measuredand reference sides of the diaphragm varies, the diaphragm will flextowards or away from the reference pressure. This flexing results instrain applied to the strain gauge which in turn provides anelectrically measurable indication of the degree of strain. The degreeof strain is then hopefully proportional to the pressure difference.Strain gauge pressure measurement has the advantage of providing adirect electrical signal that can be readily monitored by automaticequipment such as computers or controllers. Strain gauge meters can alsobe made much smaller than Bourdon tube devices and can even bemicromachined into integrated circuit wafers. Disadvantages includetemperature induced variation in response, poor unit to unitrepeatability and high cost.

Optek Technology, Inc. of West Crosby Road, Carrollton, Tex. 75006 has apublished data book (copyright 1989, 1990) that indicates its OPB modelXXX series sensors can be used to sense the absence or presence of aliquid. FIG. 5 of Optek Application Bulletin 204 (July 1989) notes thatlight signal variations due to reflection at an interface between aliquid and a transparent material can be used to detect the presence ofa liquid.

In laboratory settings it is possible to indirectly determine gasdensity or pressure by monitoring variations in the speed of sound orspeed of light in the gas. However, until now this has not translatedinto a low cost, practical and readily employable device outside thelab.

DESCRIPTION OF THE PRESENT INVENTION

The present invention utilizes a variation in index of refraction (ratioof light speeds) with respect to a chemical or physical property of afluid. This variation is exploited by placing a refracting and/orreflective device such as a prism in a light path so that the lightreflects from an interface between the reflective device and a fluidwhose chemical or physical property is to be measured. The amount oflight refracted or reflected varies as the index of refraction of thefluid in contact with the device varies. This variation in light canthen be measured by devices such as photodiodes or photocells and usedto determine the chemical or physical property.

Apparatus constructed in accordance with a preferred embodiment of theinvention monitors a density of a gas contained within a vessel. Thevessel includes a gas port for putting gas into the vessel and at leastone optics port for allowing radiation to enter the vessel. Amulti-sided radiation transmissive prism is supported within the vesselto intercept radiation entering the vessel and includes first and secondreflecting surfaces at an interface between the gas and the radiationtransmissive prism. A radiation source directs a beam of radiation intothe vessel through the optics port to the radiation transmissive prismalong a path that causes the radiation to strike a first reflectingsurface at a controlled angle and then reflect off the first reflectingsurface and pass through the prism to the second reflecting surface. Adetector monitors radiation intensity after the radiation reflects offthe second reflecting surface and provides an indication of gas densitybased upon an output from the detector.

Challenges include locating cost effective and temperature stable lightsources and light measuring devices as well as designing and buildingoptical components that are sensitive to small variations in index ofrefraction. These and other challenges are overcome in the preferredembodiment of the present invention which is described in greater detailbelow in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of a gas density sensor constructed inaccordance with the present invention;

FIG. 2 is a graph showing light emitting diode output intensity as afunction of angle;

FIGS. 3 and 4 are schematic depictions of circuits for energizing alight emitting diode and monitoring signal outputs from two photodiodes;

FIG. 5 is a graph showing theoretical sensed light intensity as afunction of gas density based upon derived equations;

FIG. 6 is graph showing experimental test data confirming thetheoretical plot of FIG. 5;

FIG. 7 is a section view of a sensor connected to a vessel containing afluid whose density is determined;

FIG. 7A is a section view of a sensor body for supporting a gas densitysensor constructed in accordance with the present invention;

FIG. 8 is a plan view of the FIG. 7A sensor body;

FIG. 9 is a plan view of an optical subassembly of the FIG. 7 sensor;

FIG. 10 is a view as seen from the plane 10--10 in FIG. 9;

FIG. 11 is a plan view of a connector for routing electric signals intothe sensor body of FIG. 7A;

FIG. 12 is a section view of the FIG. 11 connector;

FIG. 13 is a plan view of a circuit board supporting electronics foractivating the gas density sensor;

FIGS. 14 and 15 are plan and elevation views of a clip for holding aglass prism inside a pressure vessel;

FIG. 16 is a schematic representation of an alternate fluid densitysensor;

FIGS. 17A-17C are schematics of multiple alternate ways of prismmounting configurations;

FIGS. 18A-18C illustrate additional alternate prism and radiation sourceconfigurations;

FIG. 19 is an alternate embodiment of the invention that uses a singleentrance and exit light path;

FIGS. 20A and 20B are more detailed schematic diagrams of circuitry foractivating a light emitting diode;

FIG. 21 is an alternate embodiment of the invention that incorporates aglass rod rather than a multifaceted prism as a means for determiningindex of refraction of a fluid in contact with the rod;

FIG. 22 is a section view showing an alternate means of mounting a prismin a vessel;

FIGS. 23 and 23A are section and plan views of a means of mounting aprism using a glass preform that supports the prism; and

FIGS. 24 and 24A are section and plan views of a means of supporting aconical prism within a vessel.

BEST MODE FOR PRACTICING THE INVENTION

The physical principle behind the invention comes from the ClausiusMossiti Relation. ##EQU1##

For instance, in Argon the standard temperature and pressure densityis 1. 7832×10⁻⁶ g/cm³ and the STP permitivity is 1.000517. This gives avalue of 0.097 for alpha. Equation 1 implies that the permitivity of agas arises primarily from the individual dipole moments of theconstituent molecules. As a result, the greater the density of the gas,the greater the number of dipoles (molecules) in a given volume of gaswhich increases the value of the permitivity. In the appropriate unitsthe permitivity is given by equation 2. ##EQU2##

Equation 3 for electric susceptibility as a function of density is thenobtained from equations 1 and 2. ##EQU3##

If the susceptibility (X) is small, one can apply the binomial expansionto equation 3 expand the denominator. If the susceptibility is less than0.01 one can truncate to only linear terms and obtain the following:##EQU4##

Keeping only the first order term one has equation 5. ##EQU5##

Substituting the definition of susceptibility from equation 2 therelation becomes ##EQU6##

This implies that for gasses with a permitivity no greater than 1.01,the permitivity (ε) will vary linearly with density to 99% or better.For most if not all gasses the relationship between susceptibility andthe index of refraction n for the gas is: ##EQU7##

Substituting for the susceptibility and using the binomial expansion(first 3 terms) the following is obtained: ##EQU8##

For small susceptibilities the non linear term can be dropped to obtain:##EQU9##

This shows that the index of refraction n will be linear to better than98% for susceptibilities <0.01. Snell's Law states that for two mediawith indexes of refraction of nl and n2, respectively: ##EQU10##

In this relation, the angles are measured relative to the normal to theinterface between the two media and indicate the trajectory a light raywill follow in going from one media to the next. For n1>n2, it ispossible to set the angle ⊖2 to 90 degrees to obtain the critical angle:##EQU11## Substituting for n2 from equation 10 the variation in criticalangle with respect to gas density is shown as: ##EQU12## When thedensity term is sufficiently small, the critical angle will varyapproximately linearly with density.

At angles less than the critical angle, partial reflection will occur.The vector component of light polarized perpendicular to the plane ofthe incident, reflected and refracted rays, can be determined as can thevector component of light polarized coplanar with the incident,reflected and refracted rays. With n_(g) being the index of refractionof the gas, the fraction of co-planar polarization light is given byequation 13: ##EQU13## Where n is the index of refraction on theincident media.

For a non-polarized light source, an approximately equal mixture ofco-planar and perpendicular components can be expected, this results inan average reflected to incident power ratio as given by equation 14:##EQU14##

Note that the index of retraction n_(g) will be relatively independentof temperature as long as the dipole moments of the individual moleculesin the gas remain constant. For instance, monoatomic molecules such asin gaseous Argon could be expected to have relatively stable dipolemoments over temperature. More complicated molecules such as H₂ O mightbe expected to have vibrational modes that would affect the dipolemoment with increasing temperature.

FIG. 1 schematically depicts a sensor 10 that monitors gas densitychanges by monitoring changes in the index of refraction of the gas. Apressure vessel 11 includes two glass optical ports 12, 14 and a prism16. The pressure vessel defines a vessel interior 18 that contains apressurized gas mixture of 95% Ar and 5% He.

The prism 16 is composed of crown glass with an index of refraction of1.523. The prism angles are as shown in FIG. 1. The prism angles arechosen to produce controlled optical paths for light entering the prismwhen the gas density inside the vessel 10 is that found at 20° C. and3,100 psi. A principle ray 26 passing through the port 12 enters theprism 16 at an angle normal to a prism base 28 and strikes a surface 30at the critical angle of 43.7°, as determined by equation 12 (withassumptions concerning the gas mixture and gas density at 20° C. and3,100 psi) and is totally reflected. A reflected principle ray 32 passesthrough the prism 16 and strikes a prism surface 34 at the criticalangel (approximately 43.7°) and is again totally reflected along a path36. The reflected principle ray 36 then exits the prism through the base28 and leaves the vessel 11 by the exit port 14.

A light ray 42, 44, 46 strikes the prism surface 30 at an angle lessthan the critical angle and is partially reflected at the surface 30according to equation 14 and will be totally reflected at the surface34.

A light ray 52, 54, 56 that strikes the prism surface 30 at an anglegreater than the critical angle will be totally reflected at the prismsurface 30 and will be partially reflected at the surface 34.

As the gas density rises above the 20° C., 3100 psi density the index ofrefraction n_(g) of gas within the vessel interior 18 will increase aswill the critical angle (via equation 12) and all light rays enteringthe prism 16 through the base 28 will be partially reflected at one orboth of the prism surfaces 30, 34. When the gas density falls below the20° C., 3100 psi density, the index of refraction and critical anglewill decrease resulting in the principle ray 26, 32, 36 being totallyreflected, at both prism surfaces 30, 34. For rays that are partiallyreflected, the power fraction of light reflected will be given byequation 14. As the gas density continues to fall, rays with anglesclose to those of rays 26, 32 and 36 will progressively begin to go intocomplete internal reflection at both faces of the prism. As a result,the amount of light making reflections will increase with decreasing gasdensity.

In the preferred embodiment of the present invention, an infrared LED60, (Hamamatsu part L3080) is placed in front of the port 12 to shinelight through the port and into the prism 16. An infrared photodiode 62,(Hamamatsu part S2506) is placed in front of the port 14 as shown and asecond `reference` infrared photodiode 64, (Hamamatsu part S2506) isplaced near the first LED 60 as shown. The LED 60 has an intensity vs.emission angle relationship such as is shown in FIG. 2.

The dimensions and separation of the ports 12, 14 limit the emissionpattern that reaches the prism surfaces 30, 34 and passes through theexit port 14. Ideally, the emission pattern for the selected LED 60would be sharply peaked at the center line to a region offset by 5degree on either side of the centerline. However, in practice such tightemission patterns involve higher LED costs and require tighter assembletolerance. By using a comparatively broad emission pattern such as shownin FIG. 2, the prism 16 remains well illuminated even if the LED 60 isinstalled with 5° of angular misalignment. As shown by the equationsdescribed above, the fraction of light beam reflected through the prism16 back to the detector 62 is a function of the dielectric density ofthe gas under test, which is in turn strongly dependent on the gasdensity. However, the absolute strength of the light beam and as aresult, the total amount of light reaching the photo detector 62, willdepend directly on the amount of light emitted from the LED 60.

Emissions from an LED can vary with time (often decreasing), temperatureand the amount of current drive. For instance, over a temperature rangeof -40° C. to +85° C., a LED's light emission can vary by 50%. Overtime, a LED's light emission can decrease to 80% or less of its initiallevel. Also, to a large extent, an LED's light emission will varyproportionally with drive current.

In contrast, the light current from a photodiode will be comparativelystable over time and temperature at a constant level of illumination.Additionally, light current from a photodiode will vary linearly withrespect to the amount of illumination it receives over a broad range ofillumination levels (assuming the illumination is in the photodiode'sspectral range of sensitivity and does not vary in its spectraldistribution). This comparative stability of photodiodes relative toLEDs is exploited by the use of the reference photodiode 64.

The reference photodiode 64 is placed so it is illuminated by lightcoming directly from the LED 60 along a light path 66. Light passingthrough the port 12 to the prism 16 and out the port 14 that reachesphotodiode 62 will depend on the gas density (via equations 1-14) and onthe amount of light emitted from the LED 60. Light reaching thephotodiode 64 will depend only on the amount of light emitted from theLED 60. As a result, the ratio of the light currents from thephotodiodes 62, 64 cancels out dependence on the amount of LED emissionand varies primarily with gas density, i.e., if the LED 60 lightemission increases by 30%, the light currents from the photodiodes 62,64 will both increase by 30%.

In the preferred embodiment, the light current from each photodiode 62,64 is amplified to produce a voltage that varies directly with lightcurrent and amplifier gain as shown in the simplified schematic in FIG.3. When the sensor 10 is assembled, the gains of two amplifiers 70, 72are adjusted to produce a preselected voltage `Vref` from the referencephotodiode and a preselected voltage `Vsig` from the signal photodiodeat a preselected calibration gas density `Deal` at a calibrationtemperature `Teal`.

The LED 60 is driven in one of two ways (see FIG. 4). In method A, thevoltage across a resistor 76 in series with the LED 60 is kept constant.This results in a constant current drive mode for the LED 60. In methodB, a Vref signal 78 from the reference photodiode 64 is fed back into anLED drive amplifier 80 which in turn alters Vdrive to keep Vref equal toa control voltage Vset 82. This effectively keeps the light output fromthe LED 60 constant despite variations in its emission or emissionefficiency with respect to changes due to an otherwise variable drivelevel, temperature or aging. Method B is the preferred mode if theoutput Vsig 84 is going to be used as an analog indication of gasdensity. Method A is useful when all that is required is a `greaterthan` comparison between Vsig 84 and Vref 78 for switchpoint gas densitydetection. It is also useful in situations where overall current usagemust be controlled.

Given an angular acceptance of about 5° and the arrangement of FIG. 1,the fraction of the LED beam that can be expected to reach the photodetector 62 is shown as a function of gas density in FIG. 5 (curve isfor pure Argon). Note the curve is normalized relative to the fractionpredicted at a density of 0.0017832 g/cm³ (20° C. and 760 mmHg density).As can be seen, there is an approximately linear relationship betweenthe beam fraction and density up to the critical density of 0.35 g/cm3.Thereafter the beam fraction shows a slow exponential decrease.

The linear region, 0.0018932 to 0.35 g/cm³, corresponds to the densityrange where some fraction of the central ±5° of LED beam is totallyreflected at prism surfaces 30, 34. As shown in equations 1-12 for asmall electric susceptibility X, the relationship between critical angleand gas density is approximately linear. The portion of the ±5° beamemission that is totally reflected through the prism can then beexpected to be approximately linear in gas density. Since the totallyreflected beam component will have a much higher energy than thepartially reflected component (see equation 14), an approximately linearrelationship between the total amount of light reflected through theprism 16 and gas density is seen. Above the 0.347 g/cm3 gas density, theonly beam components that make it through the prism are those that arepartially reflected from prism surfaces 30, 34. Referring to equations10, 12 and 15, one sees that little linearity in partial reflections vs.density are expected.

At a constant temperature the gas density would be expected to varyapproximately linearly with pressure. In FIG. 6, normalized beamfraction reaching the photo detector 62 vs. gas density data is shownfor pure Argon gas and a prism 16 and pressure vessel 11 constructedaccording to FIG. 1. As can be seen, the relationship is approximatelylinear up to the highest pressure. A strong and monotonically decreasingrelationship between beam fraction and density is shown. This is ingeneral agreement with the type of behavior anticipated by the precedinganalysis. Note that in FIG. 5, the predicted beam fractions areapproximately 0.8, 0.65 and 0.4 at gas densities of 0.12, 0.2 and 0.27respectively. In FIG. 6, experimental data show that the actual beamfractions at the same densities are 0.81, 0.6 and 0.4 respectively. Thisshows a good agreement between theory and practice for the sensor 10.

A major advantage of the invention is the fact that the light signalfollows the unchanging path of the optics without physically contactingthe gas under test. As long as the prism surface is "wetted" by the gaswithin the vessel 11, the light signal should be relatively immune todust contamination or variations in gas color and light absorptioncharacteristics. Additionally, the reflection effect in the prism isaffected by the gas density, not the gas temperature or pressure. As aresult, the sensing effect is highly stable against changes intemperature and pressure at the same gas density. On the other hand,knowledge of temperature and gas law for a given gas allows the densitymeasurement to also be used in determining pressure.

FIGS. 7, 7A and 8 depict a sensor 110 constructed in accordance with apreferred embodiment of the present invention. The sensor 110 is coupledto a vessel 112 having a necked down end 114 that engages the sensor110. A sensor body 116 defines a fill hole 117 which extends through acylindrical post 118 of the body 116 and opens into an interior 120 ofthe vessel. The fill hole 117 allows fluid whose density is underdetermination to be injected into the vessel 112.

The sensor body 116 is formed from metal and includes a cavity 122 intowhich an optical subassembly 130 and printed circuit board 131 (FIG. 13)are inserted. During fabrication of the sensor 110, a prism 132 is fixedto the sensor body 116 in a position that covers two glass-filledoptical ports 134, 136 in the sensor body 116. The optical ports 134,136 transmit light from a light-emitting diode 60 mounted to the opticalsubassembly 130 into the prism. The prism has surfaces 132a, 132b indirect physical contact with a gas within the vessel interior 120.

A cavity or recess 140 is machined in the sensor body 120 so that afterthe prism 132 is attached to the sensor body the prism is protected fromdamage. During shipment of the sensor 110, a removable protective boot(not shown) also covers the prism 116. A metal clip 141 (FIGS. 14, 15)is also attached to the sensor body 120 to prevent movement of the prism132 if the attachment between the prism 132 and sensor body 116 shouldloosen.

The optical subassembly 130 is depicted in greater detail in FIGS. 9 and10 of the drawings. A well 142 in the sensor body extends beneath a base144 of the cavity 122. The well 142 is shaped to receive the opticalsubassembly 130 as the sensor is being built. When inserted into thewell, the subassembly 130 positions the LED 60 in a region 146 so thatlight emitted from the LED 60 passes through the optical port 134 andinto the prism. When mounted to the subassembly 130 the LED 60 abuts anexit aperture 152 (FIG. 10) that opens into a passageway 154 leading tothe optical port 134.

A carrier member 150 defines two annular bosses 160, 162 that extendoutwardly from a bottom surface 164 of the carrier 150. These bosses160, 162 seat within counterbores 166, 168 in the sensor body and helpposition the carrier 150 as the sensor 110 is assembled.

The optical subassembly 130 also supports two photodiodes 62, 64 formonitoring light emissions from the light-emitting diode 60. A referencephotodiode 64 is positioned within a cavity 170 so that a smallpercentage of the light generated by the light-emitting diode 60 passingthrough a passageway 172 reaches the photodiode 64. The secondphotodiode 62 is positioned in a cavity 174 to monitor light passingthrough the optical port 136 that has been twice reflected at the twosurfaces 132a, 132b between the prism and gas within the vessel interior120. By monitoring a ratio of the outputs of these two photodiodes 62,64, a determination of the fluid density widen the vessel can be made.

As seen :most clearly in FIGS. 14 and 15, the clip 141 is a stamped flatmetal piece bent along its length and including a mounting opening 176to allow a connector 177 (FIGS. 7, 7A) to connect the clip 141 to thesensor body. A second opening 178 overlies a prism apex and assures theprism will not separate from the sensor body.

Circuitry 180 (FIGS. 20A, 20B) for activating the light-emitting diode60 and monitoring output signals from the two photodiodes 62, 64 issupported on the printed circuit board 131. An opening 182 in thecircuit board 131 fits over the column 118 in the sensor body 116 toallow the board to be placed into the recess 122 in the sensor body 116.

Two capacitors 183a, 183b and two diodes 184a, 184b provide input noisefiltering and overvoltage protection to the circuit 180. The capacitor183a provides a low impedance path across the input terminals 185a, 185bto shunt high frequency noise around the circuit 180 and protect againstelectrostatic damage. The diode 184a prevents reverse polarity currentflow. The capacitor 183b acts as a DC noise filter and the diode 184bprovides overvoltage protection by clamping the voltage across inputs185a, 185b to the circuit to a maximum value.

A transistor 186 and biasing resistors 187a, 187b form a solid stateswitch 188 for providing power to the circuit 180. The resistor 187alimits current to the base of the transistor 186 and the resistor 187bprovides pull-up voltage to the base when the transistor 186 turns off.

The light-emitting diode 60 is coupled to the collector of a transistor190. When the transistor 190 conducts, the diode 60 conducts and emitsan infrared output. A precision regulator 192 (Texas Instrument's PartNo. TL431IPK) maintains a constant current through two resistors 194,195 by controlling the base current through the transistor 190. Currentat the base of the transistor 190 creates collector-emitter current flowthrough the transistor 190 to develop a voltage drop across theresistors 194, 195. The voltage generated across the resistors is usedby the precision regulator 192 to control base current of the transistor190. The collector to emitter current through the transistor 190 is thesame current that flows through the infrared emitting diode 60. Aresistor 196 acts as a current limit and series voltage drop for theregulator 192.

Signals developed by the two photodiodes 62, 64, in response to aninfrared light output from the light-emitting diode 60, are applied totwo precision operational amplifiers 198, 199. Light striking thedetector 62 generates current flow from the cathode to the anode of thedetector. The output voltage developed by the operational amplifier 198increases until the current through a pair of factory settable basedupon circuit performance feedback resistors 200a, 200b is equal to thecurrent through the diode 62. A capacitor 201 filters high-frequencynoise from the gain loop of the operational amplifier 198.

In a similar fashion, the current through the diode 64 generates currentflow from its cathode to anode. The voltage developed at an output fromthe operational amplifier 199 increases until the current through theseries feedback resistors 202a, 202b equals the current through thediode 64. A capacitor 203 filter high-frequency noise from the gain ofthe operational amplifier 199.

The output from the operational amplifier 199 is a signal related to thestrength of the output from the light-emitting diode 60 without theaffect of gas density. This output is coupled to two comparatoramplifiers 204, 206 (FIG. 20B), having outputs coupled to two latchcircuits 208, 210. An output from the operational amplifier 198 is alsocoupled to the comparator amplifier 204. The comparator 204 compares thevoltage signal output from the reference photodiode 64 with the signaloutput from the receiver photodiode 62. Two resistors 211a, 21lb at theinputs to the comparator 204 prevent overloading of those inputs. Afeedback resistor 212 provides positive feedback and preventsoscillation of the output from the comparator amplifier 204.

In a similar fashion, two input resistors 213a, 213b prevent overloadingof the comparator 206. A feedback resistor 214 produces a positivefeedback and prevents oscillation of the output from the comparator 206.

The latch circuits 208, 210 control application of power to thelight-emitting diode 60. When the inverted output 214 (not Q) from thelatch 208 allows the base of the transistor 186 to go high, thelight-emitting diode 60 is extinguished. When this occurs, the latch 210receives a clock input and prevents the latch 208 from again turning onthe light-emitting diode 60. If the signal from the comparator 198 isless than or equal to the signal from the comparator 199, the outputfrom the comparator 204 will be high.

A power-ON reset function is supplied by a transistor 215 and a chargingcircuit that includes a capacitor 216. The transistor 215 turns on whenpower (VCC) is applied to the circuit 180. As the transistor 215conducts in response to the power input, a voltage drop develops acrossa resistor 217 coupled to the non-inverting (+) input to the operationalamplifier 198. This positive saturation of the operational amplifier 198causes the comparator 204 to output a positive or high-level signalwhich sets the outputs of the two latch circuits 208, 210. When thelatch 208 is set, its inverted output (not Q) goes low, causing thetransistor 186 to conduct causing the light-emitting diode 60 to producean output signal.

After a 15-millisecond delay, the capacitor 216 has charged to the pointthat the transistor 215 is rendered non-conductive. This allows theoperational amplifier 198 to produce a valid signal from the photodetector 62. Stated another way, for the first 15 milliseconds thatpower is applied to the circuit 180, no valid signal is generated.

An operational amplifier 218 has a reference input (+) coupled to aresistance divider formed by the resistors 194, 195. This operationalamplifier 218 inhibits operation of the delay circuit signal output fromthe operational amplifier 219. If the signal at the junction between thetwo resistors 194, 195 falls below a certain level, voltage applied tothe circuit is too weak for continued operation and the latch delaycircuit is inhibited.

When power is applied to the circuit at the input 185a, an output from acomparator 219 goes low. At the same time, the transistor 215 turns on,making the output of the operational amplifier 198 go high. This highoutput is coupled to the operational amplifier 204 and causes the outputfrom this operational amplifier to set the latch circuits 208, 210. Thisturns on the transistor 186 and supplies power to the light-emittingdiode 60. As the capacitor 216 charges, the transistor 215 turns off,allowing the operational amplifier 198 to monitor signals derived fromthe photodiode 62.

An output from the operational amplifier 218 stays low as long as thesignal strength from the reference photodiode operational amplifierremains higher than the voltage from the reference input voltagesupplied by the voltage divider formed by the resistors 194, 195 of thelight-emitting circuit. The output from the comparator 218 goes high,however, if the reference voltage from the resistors 194, 195 is nothigh enough. When the output from the comparator 218 goes high, itinhibits the output from the comparator 219. If the reference signal atthe junction of the resistors 194, 195 is high enough, the output fromthe comparator 219 goes low at applied power and switches to highapproximately 100 milliseconds after power is applied to the circuit.The transition of the output from the comparator 219 from low to highthat latches the "good or bad" reading of the sensor once power isapplied. If the output from the comparator 198 that is applied to thecomparator 204 is higher than the reference input to the comparator 204,the comparator 204 outputs a high signal which overrides the inputs tothe latches and keeps their outputs in their set state. This output goeshigh if the output from the signal receiver is low, indicating a faultin the circuit 180.

The circuit 180 is tied to a monitoring circuit (not shown) whichdetermines the density of the gas by monitoring current passing throughthe transistor 186 and LED 60. If the sensed density is below athreshold value, the light-emitting diode 60 will remain on and themonitoring circuit will sense a current draw of approximately 50 mA bythe circuit 180. So long as the gas density stays above a specifiedlevel, indicating a certain amount of gas remains within the vessel, a5-milliamp (approximately) quiescent current is drawn by the circuit180.

The preferred use of the sensor is in an automobile where a Helium-Argongas within an air bag system is checked each time the automobile isstarted. The external monitoring circuit checks the gas density and, iftoo low a density is sensed, the motorist is warned that the air bagsystem needs maintenance.

The first two circuit inputs 185a, 185b to the circuit 180 are locatedoutside the sensor body 116. Leads enter the body through a plasticconnector 240 (FIGS. 11 and 12). The circuit board 131 is placed intothe cavity 122 and rests against a recessed ring 241 of the body 116(FIG. 8). The connector 240 is placed into the cavity 122 so that twoopenings 242a, 242b of a connector flange 243 align with two tabs 244a,244b in the ring 241. When seated against the tabs, the connector 240extends slightly above an upper rim 245 of the body 116.

Once the connector 240 has been attached to the body with suitableconnectors, a region between the connector 240 and an inner wall 246 isfilled with a potting compound P. A throughpassage 247 in the connectorleaves regions of the circuit exposed for attachment of leads to thecircuit board 130. These leads are carried by a harness (not shown)connected to a source of an input voltage. The harness includes a tabthat fits into an opening 248 in the connector that prevents rotation ofthe wiring harness connector.

Alternate Embodiment No. 1

An alternate embodiment of the invention is shown in FIG. 16 where asensor assembly 220 includes a light source 223 (laser, laser diode,lamp or LED), a collimating element 224 (lens and/or aperture stopassembly), two optical ports 225, 226, a prism 227, a light collector228, a photodetector 229 (photodiode, phototransistor, photocell etc.),and a pressure vessel 230 having an interior 231 filled with a gas, isshown in FIG. 16. The collimating element 224 and source 223 are alignedso that the right edge of the beam 232 strikes a prism face 233 at thecritical angle for a reference gas density (nominally the gas densityobserved at 760 mm and 20° C.).

When the gas inside the vessel 230 is at or below the reference density,all of the beam 232 will be totally reflected at the prism face 233 andwill strike a prism face 234 and be absorbed by a black coating 235.When the gas inside the vessel 230 rises in density above the referencegas density, the right most portion of the beam 232 will begin to bepartially refracted. A refracted portion 236 of the beam will thenstrike the light collector 228 and be directed through the port 226 tothe detector 229.

As the gas density rises, more and more of the beam will be partiallyrefracted and be directed to detector 229. This will result in amonotonically increasing signal with increasing density.

The functional relationship between the signal from the detector and gasdensity are optimized towards a desired behavior by selection of beamwidth and reference gas density. Advantages include a signal thatincreases with increasing gas density as opposed to decreasing withincreasing gas density (as in the preferred embodiment). As in thepreferred embodiment, instability in the light source 223 can becanceled out with the use of a reference detector 238. The choices ofelectronics will be driven by the type of light source andphotodetectors that are selected. However, in general, an amplifierwould be provided for each photodetector 229, 238. The light sourcecould be driven in constant current or constant light emission mode in afashion analogous to that described for the preferred embodiment andshown in FIGS. 3 and 4.

Although not explicitly shown in FIG. 16, a useful deployment for thisalternate embodiment is to separate the portion of the pressure vesselcontaining the ports, prism and electro-optics. This portion is thenprovided as a threaded or weldable insert that can be installed in anysuitably ported pressure vessel.

Further Embodiments and Applications

Nominally, glass and plastic optics will have indices of refraction onthe order of 1.5 or greater. Pressurized gasses such as Argon in the14.22 to 4600 psi range at 20° C., can be expected to have indexes ofrefraction that are in the 1.000 to 1.1 range. A liquid such as water orchemical mixtures with water as a solvent, can be expected to haveindices of refraction on the order of 1.33. Other liquids such asCineole, Aniline and benzine have indices of refraction of 1.456, 1.584and 1.498 respectively.

The devices described in the preceding embodiments would have readyapplication to measuring the density of other gasses besides Argon andHelium. They could also be used to determine the relative concentrationsof two gasses of different molecular dipole moments at a given pressureand temperature. In this technique a gas of higher dielectric constantis mixed with one of lower. The overall index of refraction would thenvary with the relative concentrations of the mixed gasses.

Given a liquid solvent of known index of refraction such as Water,Cineole, Benzine or even Oil, the disclosed technique could be used tomeasure the relative abundance of any chemical dissolved in or mixedwith the solvent, provided the chemical that was added effected theindex of refraction. Devices based on the preferred embodiment would noteven require transparency in the tested solution as the device issensitive to index of refraction of the tested medium and not itstransparency. Applications could include measuring the antifreezeconcentration in a radiator, gasoline contamination in engine oil,battery and concentration and contaminants in other chemicals.

In instances where the angle of total reflection is large, multifacetedprisms 250, 252, such as those shown in FIGS. 17A, 17B, would be used.In cases where the index of refraction in a liquid is sensitive totemperature, pressure, aging or some other parameter, the embodimentsdescribed could be used to measure that parameter as well.

Embodiment for High Index of Refraction Media

For substances with an index of refraction greater than that of theoptics material an arrangement such as is shown in FIG. 17C could beused. Here, a prism 254 has facets on the *inside of an otherwise solidblock of dielectric material. The medium trapped within the prism cavitywould then function as the prism. A disadvantage of this method would bethat the signal (reflected) light would have to pass through the testedmedium and would be susceptible to changes in transparency as well asindex of refraction.

Single Light Path Embodiment

In all the previous embodiments, two optical ports are shown. This isnot a requirement. A major advantage of optics resides in the fact thata "light pipe" can conduct both forward and backwards at the same time,i.e. a "light circuit" can be achieved without a circular path. Anembodiment exploiting this behavior of light is shown in FIG. 19. Alight source 337, photodetector 338 and prism 339 are optically linkedby a single light pipe 340 which in turn may be optionally provided withan optical cladding 341. Optical couplers 342 and 343 are used to linkthe light source 337 and photodetector 338 with the light pipe 340.

The source coupler 342 is shown as injecting light at a higher point andangled away from the detector coupler 343 to prevent cross-talk betweenthe couplers. A housing 344 and the cladding 341 serve to optically andphysically insulate the light path. Light from the source 337 passes upthe coupler 342 and light pipe 340 to the prism 339 where it must maketwo reflections (as in the preferred embodiment) in order to return backdown the light pipe 340, out the coupler 343 and back to the detector338. Some fraction of the reflected light that comes back down towardsthe detector will be lost down the source coupler 342.

The amount of light that is lost in the FIG. 19 embodiment will be astable fraction of the total amount that is reflected down the lightpipe 340 and can be safely ignored. In this embodiment the light pipe340 and cladding 341 are shown passing through a header 345 which inturn is sealed into a suitably sized orifice in the vessel 346 thatcontains the medium 347 that is being tested. A major advantage of thisdevice is that the light pipe 340 can be all or part of an extendedoptical fiber which would allow the electro-optics to be remotelylocated from the vessel 346 which could then be placed in an environment(such as extreme temperature) that the electro-optics could notwithstand. As in the previous embodiments, a reference detector 348 canbe used to cancel out variations in light source 337 output due toaging, temperature, variations in power supply, etc.

Use of reflections near the angle of total internal reflection serves tomaximize the sensitivity to small changes in index of refraction thatcan arise from changes in medium density, temperature, chemicalcomposition and or physical state. Prism materials can consist of any ofa number of glasses (such as crown, BK 7, fused quartz etc.) or plastics(such as polycarb). In the embodiment of FIG. 17C, the medium becomesthe prism and the block of material containing the prism defining cavityneed only have a suitably high dielectric constant and need not even betransparent. In the most similar application (see Optek Inc. applicationnote from a 1989, 1990 Data book), two reflections are used in a prismthat are in total internal reflection in a gas (air) and in almostcomplete refraction in a tested liquid (such as water). When the prismis immersed in the liquid, the amount of light making both reflectionsis abruptly and substantially reduced. In this manner the prismfunctions as a detector for the presence or non presence of liquid asopposed to gas or vacuum. No effort is made to monitor changes in indexof refraction in the gas or liquid.

Multiple alternate embodiments of the invention are disclosed in FIGS.18A, 18B and 18C. In FIG. 18A, both the light-emitting diode 60' andphotodiode 62' are supported within a prism 16'. A single reflectingsurface 30' is in contact with a gas within a vessel interior 18' ofFIG. 18A and a second reflecting surface 341 is in contact with the gasin FIG. 18C.

In FIG. 18B, the vessel 11' supports a multi-faceted prism 16". A singlereflecting surface 30" directs light to a photodetector 62". FIG. 18C issimilar to the FIG. 18A embodiment except that two reflecting surfaces30', 34' intercept the light beam as it travels from the light-emittingdiode to the detector.

Referring to FIG. 21, a cylindrical rod 400 of optically transmissivematerial (such as glass) replaces the prism of the previous embodiments.Art optical detector 402 backed by an aperture stop 404 is situated at alow pressure (or non-immersed) end of the rod 400 so that a receptiveportion of the detector 402 faces one end of the rod.

An optical source 406 supported by an insert 407 is situated below theaperture stop 404 and is oriented to direct radiation along an angled orbeveled surface 405 of the aperture stop 404 through a ring lens 408 andinto the rod 400. The diameter of the aperture stop 404, the shape ofthe lens 408, and the lens' location relative to the end of the rod aswell as the location of the source 406 are controlled to allow onlylarge angle rays 410 to enter the rod 400.

The angles of rays that are accepted into the rod 400 are constrained bythe controlled parameters to be less than or equal to the angle of totalreflection θ between the cylindrical outer surface 412 of the rod andthe medium within a vessel 413 that supports the rod. This angle oftotal reflection is of course dependant on the index of refraction ofthe test medium which is the index of refraction R at the condition ofinterest, i.e, a given density or composition.

Rays that would strike the surface 412 at angles greater than the angleof total reflection θ are stopped by the aperture stop 404. Rays thatare at or below the angle of total reflection will be totally orpartially reflected each time they strike the outer surface of the rod.

By making multiple reflections the rays 410 that enter the rod 400 willeventually reach a far end of the rod where they are reflected by areflective coating 414 applied to the far or distal end of the rod. Therays will then reflect off the coating 414 and reflect back down therod. A certain percentage or proportion of the rays 410 that are emittedto enter the rod will strike the detector having made multiplereflections at the medium/rod interface.

Rays that are only partially reflected at this interface will besubstantially attenuated if they make multiple reflections. The rays intotal reflection will suffer little attenuation as they reflect off thecylindrical wall of the rod. When the condition of the medium thatcontact the rode changes (for example when the gas density changes) tocause the index of refraction of the medium to fall below R, rays closeto the angle of total reflection will begin to go into total reflectionand the light signal reaching the detector 402 will increase. As theindex of refraction continues to drop a progressively greater fractionof rays will go into total reflection resulting in an even great lightsignal.

The rod and detector/source assembly are supported by the vessel 413containing the medium for testing. FIG. 21 also uses a referencedetector 416 mounted relative to the source 406 for use in eliminatingsource or detector response variations from the determination of themedium composition. Suitable electrical signals for activating thesource 406, and responsive to the detector 402 and detector 416 aretransmitted by conductors 420.

FIGS. 22, 23, 23A, 24 and 24A depict an alternate means of positioning aprism with respect to a vessel wall 450. The vessel depicted in FIG. 22has two ports 452, 454 that are filled with a glass and direct light toa prism 456. The prism is attached to the vessel wall 450 by means of aglass preform 460. The preform 460 is a generally square (in plan)support made of a powdered glass molded to shape with a volatile binder.A preferred powdered glass is Corning 1990 which softens at around 500°C. The prism 456 is made of a higher softening point glass such as CrownGlass which softens around 730° C. When raised to a temperature above500° C., the preform 460 fuses the prism 456 to the vessel wall tosecure the prism 456 in place.

In FIGS. 23 and 24, two preforms 470, 480 support prisms 472, 482 thatseat against a shoulder 476 of the wall 450. When fused, the preform andprism extend into the cavity in the wall and have exposed surfaces 474,484 outside the vessel. Note, the prism 482 is a cylinder having acone-shaped end 486 exposed to a medium inside the vessel interior.

While a preferred embodiment of the invention has been described with adegree of particularity, it is the intent that the invention include allmodifications from the disclosed design falling within the spirit ofscope of the appended claims.

We claim:
 1. Sensor apparatus for monitoring the composition of a fluidcomprising:a) a vessel for holding a fluid and including one or moreopenings for allowing radiation to enter and to exit the vessel; b) aradiation source for directing a beam of radiation along a path thatcauses the radiation to enter the vessel; c) a prism having a radiationtransmitting surface that covers said one or more openings in the vesseland further having first and second reflecting surfaces having aspecified orientation with respect to each other wherein a firstreflecting surface is oriented with respect to an angle of incidence ofradiation passing through the prism from the radiation source so that asignificant portion of light contacting the first reflecting surface isreflected off the first reflecting surface along a path completelywithin the prism to the second reflecting surface where a significantportion of the radiation reaching the second reflecting surface isreflected off the second reflecting surface and exits the prism and thevessel; d) a first radiation detector for monitoring intensity ofradiation leaving the vessel that has been reflected off the secondreflecting surface of the prism; e) a second radiation detector formonitoring intensity of radiation from the radiation source that doesnot enter the vessel; and f) means for determining fluid compositionbased on the intensity sensed by the first and the second radiationdetectors.
 2. The sensor apparatus of claim 1 wherein the means fordetermining the fluid composition determines the fluid samplecomposition based upon a ratio of intensities sensed by the first andsecond radiation detectors.
 3. The sensor apparatus of claim 1 where thesource is an infrared light-emitting diode.
 4. The sensor apparatus ofclaim 1 wherein the first reflecting surface is oriented with respect toan angle of incidence of radiation from the source so that a significantportion of light contacting the first reflecting surface is reflectedand a significant portion is refracted when the fluid sample compositionis near a composition of interest.
 5. The sensor apparatus of claim 1wherein the vessel has an entrance opening near the source fortransmitting the radiation into the vessel and further has an exitopening for allowing radiation reflected off from the first and secondsurfaces to exit the vessel and impinge upon the first detector.
 6. Thesensor apparatus of claim 5 wherein the entrance and exit openings ofsaid vessel are filled with a light transmissive material.
 7. The sensorapparatus of claim 2 additionally comprising intensity control circuitryfor activating the radiation source and wherein an output from thesecond radiation detector is used as a control input for the intensitycontrol circuity to increase and decrease output from the radiationsource.
 8. The sensor apparatus of claim 1 wherein the sensor apparatusmonitors a density of a gas within the vessel.
 9. A method fordetermining the density of a gas comprising the steps of:a) mounting aprism in a vessel so that first and second reflecting surfaces of theprism that have a specific orientation with respect to each other are incontact with a gas inside the vessel interior and the prism covers oneor more access openings in said vessel; b) directing radiation into oneof the access openings to pass through a radiation transmitting surfaceof the prism along a path that causes at least some of the radiation topass through the prism and reflect off the first reflecting surface to apath completely within the prism to the second reflecting surface wheresaid radiation is again reflected toward one of said access openings toexit the vessel; c) monitoring radiation leaving the prism that hasreflected off the first and second reflecting surfaces; and d)correlating monitored radiation with a density of gas inside the vessel.10. The method of claim 9 wherein the step of directing radiation intothe prism is performed by positioning a radiation source to directradiation through the prism transmitting surface substantially normal toa radiation path.
 11. The method of claim 10 comprising the additionalstep of monitoring radiation intensity from the radiation source thatdoes not enter the vessel in addition to monitoring radiation intensityof the radiation that enters and exits the prism and wherein the step ofcorrelating monitored radiation intensity with fluid density isperformed by taking a ratio of intensities of radiation that does anddoes not pass through the prism.
 12. Apparatus for monitoring a densityof a gas comprising:a) a vessel containing a gas whose density is beingmonitored, said vessel including a gas port for putting gas into thevessel and at least one optics port for allowing radiation to enterand/or leave the vessel; b) multi-sided radiation transmissive prismsupported within the vessel to intercept radiation entering the vesselhaving first and second reflecting surfaces at an interface between thegas and the radiation transmissive prism; c) a radiation source fordirecting a beam of radiation into the vessel through the optics port tothe radiation transmissive means along a path causing the radiation tostrike a first reflecting surface of said prism at a controlled angleand then reflect off said first reflecting surface and pass along a pathcompletely within the prism to said second reflecting surface; d) adetector for monitoring radiation intensity after the radiation reflectsoff the second reflecting surface of said prism; and e) circuitry forproviding an indication of gas density based upon an output from thedetector.
 13. The apparatus of claim 12 where the circuitry provides atwo-state output signal indicating whether gas density within the vesselis above or below a specified gas density.
 14. A sensor for monitoringthe composition of a fluid comprising:a) a vessel for holding a fluidwithin a confined region; b) an elongated radiation transmissive rodhaving a reflecting surface at a distal end and a generally cylindricalouter surface in contact with the fluid within said vessel symmetricwith respect to a centerline of the rod that extends along at least aportion of the length of the rod; c) a radiation source positioned alonga line co-incident with the centerline of said rod for emittingradiation toward a proximal, non-immersed end of the elongated rod; d)an aperture stop positioned between the radiation source and theproximal end of the rod that includes a surface for interceptingradiation from the source to allow said radiation to enter the proximalend of said elongated rod at only controlled angles; e) a firstradiation detector mounted to the aperture stop for monitoring intensityof radiation that has reflected off the reflecting surface of the rod;f) a second radiation detector located behind the aperture stop formonitoring intensity of radiation from the radiation source that doesnot enter the elongated rod; and g) means for determining fluidcomposition based on the intensity sensed by the first and the secondradiation detectors.
 15. The sensor of claim 14 wherein the firstradiation detector is positioned near the proximal end of the rodopposite the reflecting surface and is positioned within a cavity in theaperture stop.
 16. Sensor apparatus for monitoring the composition of afluid comprising:a) a prism positioned with a fluid sample and having arefracting surface in contact with a fluid sample; b) a radiation sourcefor directing a beam of radiation along a path that causes the radiationto reach the refracting surface of the prism at an angle to a normaldirection of the surface to cause a substantial portion of the radiationto be refracted away from the normal to pass through the refractingsurface and enter said fluid; c) a first radiation detector formonitoring intensity of radiation that has been refracted at therefracting surface of the prism and enters the fluid sample; and d)circuitry for determining fluid composition based on the radiationintensity sensed by the first radiation detector.